As spectacle lenses for people whose accommodation is decreased with aging, progressive-addition spectacle lenses (referred to as “progressive lenses” hereinafter), is becoming widely used. Since there are a great variety of types of the progressive lenses designed for different usage environment and intended use of individual wearers of the spectacles, and since it is necessary to provide suitable optical performance for individual wearers according to his/her refractive error and accommodation abnormal, a great number of design conditions (such as the distance-portion power (i.e., the spherical power, the cylindrical power and the cylinder axis), the addition power, the prismatic power, the optical design type, the corridor length, the inset, the center thickness, the edge thickness, the outer diameter, and the like) need to be specified when producing the lenses, so that there will be a tremendous number of combinations of these design conditions.
Thus, it is very difficult to previously prepare all types of finished lenses (i.e., the lenses whose both optical surfaces are optically finished). For this reason, the progressive lens is typically produced on a made-to-order basis after receiving the order from the customer by processing a previously produced lens blank based on individual design conditions. For example, semifinished lens blanks each having an optically finished lens front surface (i.e., convex surface) and an optically unfinished thick lens back surface (i.e., concave surface) are previously prepared and stocked (wherein the optically finished lens front surface is a progressive-addition surface), and a most suitable semifinished lens is selected from these semifinished lens blanks according to the order of the customer. Thereafter, the back surface of the selected semifinished lens is optically finished by being cut with a cutting machine (i.e., a curve generator) and polished with a polishing machine according to the order details of the customer, and thereby a progressive lens with desired optical performance is produced (see, for example, Patent document 1).
Further, in order to reduce the thickness and weight of the progressive lens, a prism thinning process is conventionally performed to cut the back surfaces of a pair of left and right semifinished lenses so that an identical prism having vertical base direction is added to both the left and right lenses (see, for example, Patent document 2).
Furthermore, as one of methods for producing the semifinished lens blank, a cast polymerization method employing a tape molding method has been conventionally used. In such cast polymerization method, two molding dies for forming the front surface and the back surface of the lens are held at a predetermined distance from each other, an adhesive tape is wrapped around the two molding dies so as to form a lens molding die, a kind of plastic lens material liquid is poured into the lens molding die, and the plastic lens material liquid is polymerized and cured to form a lens (see, for example, Patent document 3).